This invention relates to a tubular product such as a tubular casing, and in particular to a tubular casing having a predetermined maximum extensible diameter.
The main application for tubular products such as tubular casing of the variety described in this specification is in the food industry. However, the invention is not restricted to this particular application. Tubular products such as knitted netting or elasticated knitted fabric casing are used in the cooking and curing process of meat products. In the cooking of certain meat products, it is common to pack the meat pieces into an elasticated tubular net. The tubular net compresses the meat pieces and holds them together during the cooking process.
Such tubular products are normally removed from cooked meat products which can then be further packaged, ready for sale. Elasticated tubular casing is also used on cured meat products such as ham and metwurst.
The means of filling tubular casing normally comprises fitting the casing onto a hollow cylindrical mandrill, which is known as a stuffing tube, and using a machine to pump or force meat product under pressure through the stuffing tube into the casing. The casing is drawn from the stuffing tube as the meat product is extruded out of the stuffing tube. The process is started by first clipping the end of a casing once sufficient casing is placed on the stuffing tube. Further clips are placed at intervals dependent on the required length of the product.
Such machines can be used either with minced meat products, meat portions or whole meat muscle.
It is difficult to maintain a constant diameter of the resultant product as the casing is being filled. Tubular product that is commonly used such as tubular netting is able to expand to very large diameters. The required diameter is normally achieved by controlling the pressure at which meat product is pumped through the stuffing tube. However, it is still possible to produce a filled casing that varies considerably in diameter. This is quite undesirable as many of the compressed meat products are normally sliced at the time of purchase. Therefore, it is desirable to ensure a constant diameter and to also not exceed certain diameters which would be unsuitable for some slicing machines. Clearly, a product of uniform diameter is more visually appealing and avoids wastage and reworking of product at the manufacturing stage.
Accordingly, it is an aim of the invention to provide a tubular casing which resists expansion beyond a required or preset diameter.
In its broadest form, the invention is a tubular casing for use with food product that includes,
circumferential threads along the length of said tubular casing that become taunt at a preset diameter so that said tubular casing has a predetermined diameter when filled with food products.
Preferably, the circumferential threads may comprise either elasticated threads or inextensible threads.
In the case of inextensible threads, the threads may be incorporated into either a net or netted stockinette so that, at a relaxed diameter, the circumferential threads lie loosely on the surface of the net or stockinette. However, at the preset filled diameter of the casing, the threads become taunt and prevent further expansion of the net or stockinette. In the case of elastic circumferential threads, the elastic threads are provided with an elastic limit where, below the elastic limit the threads remain extensible, and that at the elastic limit, the threads become inextensible.
Elasticated thread is normally a combination of rubber thread or strips with natural or synthetic yarns wrapped around the length of the rubber. A large number of turns of yarn per centimeter of rubber is normally used to cover the rubber surface. Elasticated threads formed in this way have more than adequate extensibility. In fact, the elasticated threads remain extensible well beyond the desired maximum diameter of the product being produced which allows bulging to readily occur.
In a further aspect of this invention, elastic thread is provided with an elastic limit by controlling the rate at which yarn is wrapped onto the external surface of the rubber thread. It has been found that the elastic limit can be set by controlling the number of turns of yarn per centimeter of elastic thread.
The yarn forms a helix or spiral around and along the length of the rubber thread. As the elastic thread is stretched, the helix or spiral expands so that the helix angle increases thereby enabling stretching of the rubber. At the same time, the diameter of the helix or spiral reduces. If the number of turns of yarn per centimeter on the elastic thread is limited, then the lengthwise expansion of the helix and its continual reduction in diameter will result in a limit being reached whereby further tensile force applied to the elastic thread is resisted by the yarn. This occurs when the yarn helix effectively approximates a straight piece of thread where stretching force results in only tensile force within the yarn.
Although the yarn wrapped around the rubber thread does not completely straighten, it will reach a point where the helix is unable to straighten any further, and therefore approximates a straight length of thread.
Once the tensile force is resisted by the yarn, the elastic thread will not be able to stretch. The elastic thread is then effectively inextensible. That is, further force applied to thread will not result in any additional stretch or extension.
The elastic limit is determined by the number of turns of yarn per centimeter around the rubber thread. The limit can therefore be predetermined, and as such tubular casing using elasticated thread can be produced which will have a maximum diameter to which it can expand.
A typical method for manufacturing tubular casing is by circular knitting machines. Typical casings are single knit jersey tubes which may incorporate circumferential and longitudinal threads. The circumferential threads normally comprise a single thread that forms a spiral that extends along the length of the tubular casing.
In addition, circular knitting machines may be used to produce tubular netting which comprises longitudinal and circumferential thread members. Again, the circumferential thread forms a continuous spiral along the length of the tubular casing. The longitudinal stitches are spaced at regular intervals around the circumference of the casing.
Accordingly, the invention may be utilised in respect of stockinette casings incorporating inextensible or elastic threads as circumferential members. Also, the invention may be used with net casing products where either inextensible or elastic threads according to the invention are used.
In addition, the invention may also be a tubular casing which is a combination of a net and fibrous casings. The fibrous casing is located within and extends along the length of the net casing. Preferably, the diameter of the fibrous casing greatly exceeds the diameter of the net casing in its relaxed form. This ensures, in the case of a net having elastic circumferential threads, that a constant compressive force is applied to meat product as it is forced into the tubular casing.
Preferably, the stuffing process results in embedding of circumferential and longitudinal threads into the surface of the meat product once the maximum diameter is reached. This will serve to produce a pattern on the surface of the meat product once the casing is removed. In the case of a combination of net and fibrous casing, the maximum diameter of the net is slightly less than the maximum diameter of the fibrous casing to ensure formation of the pattern effect.